1. Field of the Invention
The present invention relates to concentric riser joints and more particularly, to concentric riser joints with a self-aligning coupling.
2. Description of the Prior Art
A marine riser system is used to guide a drill string to the wellhead and provide a fluid flow path between the top of the wellbore, which is on the ocean floor, and the drilling equipment, located above the surface of the water. The marine riser system comprises several sections or joints of pipe and special devices to compensate for any movement of the drilling rig caused by waves.
One of the joints of pipe in the marine riser system is termed a standard riser joint. Typically, the standard riser joint is provided in 50 foot lengths. Several standard riser joints are coupled together in the marine riser system with the exact number depending on the water depth.
A concentric riser comprises an inner pipe housed inside an outer pipe with an annular area therebetween. The inner pipe serves as the primary flow conduit and also provides vertical access to the well production bore for drilling, wireline or coiled tubing intervention. The annular area between the two pipes provides a second flow path which can be used to circulate the well or otherwise communicate with an annulus bore in the subsea Christmas tree or tubing hanger.
Prior art concentric riser joints have included end couplings with passageways for the annular fluid to pass therethrough. From an operational and fluid flow standpoint, the optimum design for a concentric riser joint would have no obstructions in the primary flow path and the annular flow path. However, some obstruction in the annular flow path will always occur at the couplings of the riser joint. Additionally, the annular fluid flow disturbances should be minimized at the coupled connection between two joined riser joints.
It is desirable to have a concentric riser joint which minimizes the fluid flow disturbances in the annular flow path. It is further desirable that the concentric riser joint self-aligns with an adjoining concentric riser joint to ensure that fluid flow disturbances are kept to a minimum.